Precoder for spatial multiplexing, multiple antenna transmitter

ABSTRACT

A method of spatially precoding data includes determining a transmission rank of a communication channel and selecting a set of one or more precoding filters derived from a single generator matrix based on said transmission rank. The method also includes precoding data for transmission to a remote device using said precoding filters in a predetermined order according to a predetermined precoding sequence. The precoding may include using different ones of said precoding filters during different precoding intervals in a predetermined precoding period of the predetermined precoding sequence. Additionally, precoding data for transmission may involve traversing an Orthogonal Frequency Division Multiplexing (OFDM) resource block in an alternating pattern during said predetermined precoding period.

CLAIMING BENEFIT OF PRIOR FILED U.S. APPLICATION

This application is a continuation of International ApplicationPCT/SE2008/051155, with an international filing date of Oct. 9, 2008,which corresponds to the national-stage entry U.S. patent applicationSer. No. 12/746,448, filed Jun. 4, 2010, and which claims the benefit ofU.S. Provisional Application No. 60/991,849, filed Dec. 3, 2007, and thecontents of all of the preceding are hereby incorporated by referenceherein.

TECHNICAL FIELD

The invention relates generally to methods and apparatus fortransmitting signals using multiple transmit antennas and, moreparticularly, to methods and apparatus for spatially precoding signalstransmitted from a multiple antenna transmitter.

BACKGROUND

In recent years, there has been much interest in multiple input,multiple output (MIMO) systems for enhancing data rates in mobilecommunication systems. MIMO systems employ multiple antennas at thetransmitter and receiver to transmit and receive information. Thereceiver can exploit the spatial dimensions of the signal at thereceiver to achieve higher spectral efficiency and higher data rateswithout increasing bandwidth.

One transmission scheme for MIMO systems that is receiving significantattention is spatial multiplexing. In a spatial multiplexingtransmitter, the information symbols are precoded before transmission tomultiplex the information signal in the spatial domain. The precodingmay be channel dependent or channel independent. With channel dependentprecoding, also referred to as closed loop precoding, the precodermatrix is chosen to match the characteristics of the MIMO channel. Withchannel independent precoding, also referred to as open-loop precoding,channel characteristics are not considered in selecting the precodermatrix.

With closed loop precoding, the user equipment performs channelmeasurements on the forward link channel, and feeds back channelinformation or precoder configurations to the base station. One problemwith closed loop precoding is that it takes time to perform channelmeasurements and feed back information to the base station. During thattime, the channel conditions may have changed so that the feedbackinformation is outdated before it is used. Consequently, closed loopprecoding is typically used in low mobility situations where the channelvariations are slow.

In situations where the channel conditions vary more rapidly and lacksignificant long-term properties, channel independent precoding oropen-loop precoding may be used. With open loop precoding, the precodingmatrix is selected independent of the channel realizations. Channelindependent precoding is generally considered more suitable for highmobility situations.

One way to implement open loop precoding is to use a spatialmultiplexing precoder matrix to precode the information sequence priorto transmission. In order to accommodate a wide range of channelrealizations, it is advantageous to apply multiple precoders that arevaried in a deterministic manner known to both the transmitter and thereceiver. For example, in an orthogonal frequency division multiplexing(OFDM) system, the precoder may be kept fixed for a set of one or moresubcarriers and then changed for the next set of subcarriers. Thistechnique, referred to as precoder cycling, serves to distribute theenergy spatially in a more isotropic manner, which in turn is useful fordiversity and reducing the tendency to bias the performance of thetransmitter for a particular set of channel realizations. When applyingprecoder cycling, it is advantageous to have substantial precodingvariation over the smallest possible allocation unit, e.g., a resourceblock (RB), since a codeword may potentially only span a very small setof resource elements.

A number of drawbacks have been encountered in the past when precodercycling has been used. Interference rejection algorithms implemented atthe receiver need to characterize the spatial properties of the channelto suppress interference. It is beneficial that the interferingtransmissions have roughly similar properties over a large number ofresource elements so that averaging may be used to suppress noise andother impairments. In systems where the cycling of the precoder isconfigurable, the receiver can not be sure how fast the interferencechanges over a resource block without a priori knowledge of the precodersequence. Also, the precoders are frequently chosen from a codebookdesigned for channel dependent precoding. As a consequence, theprecoders do not distribute the energy uniformly over the vector spaceof the MIMO channel. Finally, precoder cycling increases thecomputational complexity of demodulation and CQI computation. Thecomputational complexity is bounded only by the codebook size, so thereceiver needs to be designed to handle the worst case scenario.

SUMMARY

The present invention relates to a method and apparatus for spatiallyprecoding data for transmission to a remote device over a MIMO channel.In one exemplary embodiment, the transmitter selects a transmission rankand uses a predetermined precoder sequence for the selected transmissionrank comprising one or more precoder filters. During transmission, aprecoder precodes data for transmission to a remote device usingdifferent precoding filters during different precoding intervals in aprecoding period according to the selected precoder sequence.

The invention offers an efficient way to support open-loop MIMOtransmission particularly targeting rank two or higher ranktransmissions. Computational complexity for demodulation and CQIcomputation in the UE is reduced and the feasibility of interferencerejection is improved compared to existing solutions. The increaseduniformity of the transmission in the spatial domain improves therobustness of the open-loop MIMO mode. The use of a single generatormatrix may result in considerable complexity savings as many of thecomputations for CQI and demodulation may be reused across severaldifferent ranks and when identifying the characteristics of theinter-cell interference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary MIMO channel.

FIG. 2 illustrates an exemplary transmit signal processor for an OFDMsystem.

FIG. 3 illustrates the mapping of codewords to layers as performed bythe transmit signal processor.

FIG. 4 illustrates an exemplary method for precoding data fortransmission to a remote device.

FIG. 5 illustrates an exemplary receive signal processor for an OFDMsystem.

FIG. 6 illustrates an exemplary method for receiving precoded data.

DETAILED DESCRIPTION

FIG. 1 illustrates a multiple input/multiple output (MIMO) wirelesscommunication system 10 including a first station 12 and a secondstation 14. The first station 12 includes a transmitter 100 fortransmitting signals to the second station 14 over a communicationchannel 16, while the second station 14 includes a receiver 200 forreceiving signals transmitted by the first station 12. Those skilled inthe art will appreciate that the first station 12 and second station 14may each include both a transmitter 100 and receiver 200 forbi-directional communications. In one exemplary embodiment, the firststation 12 comprises a base station in a wireless communication network,and the second station 14 comprises a user terminal. The presentinvention is particularly useful in Orthogonal Frequency DivisionMultiplexing (OFDM) systems.

An information signal in the form of a binary data stream is input tothe transmitter 100 at the first station 12. The transmitter 100includes a controller 110 to control the overall operation of thetransmitter 100 and a transmit signal processor 120. The transmit signalprocessor 120 performs error coding, maps the input bits to complexmodulation symbols, and generates transmit signals for each transmitantenna 130. After upward frequency conversion, filtering, andamplification, transmitter 100 transmits the transmit signals fromrespective transmit antennas 130 through the communication channel 16 tothe second station 14.

The receiver 200 at the second station 14 demodulates and decodes thesignals received at each antenna 230. Receiver 200 includes a controller210 to control operation of the receiver 200 and a receive signalprocessor 220. The receive signal processor 220 demodulates and decodesthe signal transmitted from the first station 12. The output signal fromthe receiver 200 comprises an estimate of the original informationsignal. In the absence of errors, the estimate will be the same as theoriginal information signal input at the transmitter 12.

Because multiple data streams are transmitted in parallel from differentantennas 130, there is a linear increase in throughput with every pairof antennas 130, 230 added to the system without an increase in thebandwidth requirement. MIMO systems have been the subject of extensiveresearch activity worldwide for use in wireless communication networksbecause of their potential to achieve high spectral efficiencies, andtherefore high data rates.

In embodiments of the present invention, the transmit signal processor120 is configured to spatially multiplex the information signal beforetransmission to realize further increases in spectral efficiency bytaking advantage of the spatial dimension of the communication channel16. FIG. 2 illustrates an exemplary transmit signal processor 120according to one embodiment of the invention for an Orthogonal FrequencyDivision Multiplexing (OFDM) system. The transmit signal processor 120comprises a layer mapping unit 122, a precoder 124, and a plurality ofInverse Fast Fourier Transform (IFFT) processors 126. The IFFTprocessors 126 may perform a Discrete Fourier Transform or the inverseoperation. A sequence of information symbols is input to the layermapping unit 122. The symbol sequence is divided into codewords that aremapped by the transmitter 100 to corresponding OFDM symbols. The layermapping unit 122 maps the codewords into one or more layers depending onthe transmission rank. It should be noted that the number of layers doesnot necessarily equal the number of antennas 130. Different codewordsare typically mapped to different layers; however, a single codeword maybe mapped to one or more layers. The number of layers L corresponds tothe selected transmission rank.

FIG. 3 illustrates the mapping of codewords to layers according to oneexemplary embodiment for transmission ranks from 1 to 4. For atransmission rank of 1, a single codeword is mapped to a single layer.For a transmission rank of 2, two codewords are mapped to two differentlayers. For a transmission rank of 3, two codewords are mapped to threelayers, and for a transmission rank of 4, two codewords are mapped tofour layers. It may be noted that the transmission rank or number oflayers need not be the same as the number of antennas. In the subsequentdiscussion, it is assumed that the transmitter 100 includes fourtransmit antennas 130.

Each layer output from the layer mapping unit 122 feeds into theprecoder 124. Precoder 124 spatially multiplexes the symbols in eachlayer by multiplying a vector s of input symbols to the precoder 124with a precoding filter. The precoding filter is an N×L matrix thatmultiplies each input symbol of the symbol vector s by a correspondingcolumn vector of the precoding matrix. In order to achieve diversity,the precoder 124 cycles through multiple precoding filters and outputs Ncoded symbol streams. Each symbol stream is output to a correspondingIFFT processor 126. In an orthogonal OFDM system, the precoding filtermay be kept fixed for a set of one or more subcarriers and then changedfor the next set of subcarriers according to the selected precodersequence. The precoding filters may be pre-stored in memory or generatedon the fly by the transmit signal processor 120 as hereinafterdescribed. The IFFT processors 126 transform the spatially coded symbolsoutput by the precoder 124 to the frequency domain to generate OFDMsymbols. The OFDM symbols output from each IFFT processor 124 are thenoutput to a respective antenna 130 via antenna ports 128 fortransmission to the receiver 200. By spatially coding the informationsymbols, it is possible to transmit multiple symbols on each resourceelement (RE) of the OFDM resource grid.

According to the present invention, precoder 124 cycles through a fixedand predetermined set of precoding filters determined based on theselected transmission rank. A precoder sequence known a priori to thebase station and user terminal specifies the set of precoding filters touse for precoding and the order in which the precoding filters in theset are applied. A different precoder sequence is defined for eachpossible transmission rank.

The precoding filters corresponding to each precoder sequence areselected to satisfy the following criteria:

-   -   the precoder sequence is the same for each resource block or        smallest resource allocation unit;    -   the precoding sequence should use the different precoding        filters an equal number of times, or as close to equal as        possible;    -   the number of different precoding filters in the precoding        sequence should be small but still distribute the subspaces        sufficiently uniform over the (complex) Grassmanian manifold;        and    -   the number of different precoding filters corresponding to one        period of the precoder sequence should be applied to resource        elements which are close to one another in the resource grid.        A precoding sequence meeting these criteria is referred to as        short uniformly varying precoding sequence (SUVPS).

In one exemplary embodiment, the precoding filters may be selected froma predetermined codebook. An exemplary codebook is the House Holdercodebook specified in the Long Term Evolution (LTE) standard currentlybeing developed. The House Holder codebook comprises sixteen precodingfilters. For each transmission rank, four of the possible sixteenprecoding filters in the House Holder codebook may be selected to form aprecoder sequence with a periodicity of four. That is, each precodingfilter is used once in one period of the precoder sequence. Theselection should be made to optimize some predetermined criterion thatstrives for a uniform distribution of subspaces over the Grassmanianmanifold according to some Grassmanian subspace packing principle. Forexample, the precoding filters may be chosen to maximize the minimumdistance between subspaces, where the distance may correspond tomeasures such as chordal, projection two-norm, or the Fubini-Studydistance.

In an orthogonal OFDM system, the precoding filter may be kept fixed fora set of one or more subcarriers and then changed for the next set ofsubcarriers according to the selected precoder sequence. To ensure thata period of the precoder sequence is localized in the OFDM resourcegrid, a precoder sequence with a periodicity of four may be applied bytraversing the resource elements (REs) in a resource block (RB) of theOFDM resource grid in a zig-zag like pattern. For example, the REs maybe traversed in a frequency first order from top to bottom in eachodd-numbered OFDM symbol period, and from bottom to top in eacheven-numbered OFDM symbol period.

In one exemplary embodiment, the precoding filters in a precodingsequence are selected from column subsets of a single generator matrix.The elements of the generator matrix may, for example, be taken from an8-PSK or QPSK alphabet. An exemplary generator matrix G for a 4 antennatransmitter is given by:

$\begin{matrix}{G = {\begin{bmatrix}1 & 1 & 1 & 1 \\1 & j & {- 1} & {- j} \\1 & {- 1} & 1 & {- 1} \\1 & {- j} & {- 1} & j\end{bmatrix}.}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$The precoding filters are derived from the generator matrix G byselecting column subsets in G as precoding filters. In order to meet therequirement for uniformity in the spatial properties, the column subsetsare selected such that:

-   -   all the columns in the generator matrix G are used an equal        number of times in one period of the precoder sequence for a        given transmission rank;    -   each precoding filter is used the same number of times in one        period of the precoder sequence; and    -   the maximum possible period length is equal to the number of        different column subsets for a given transmission rank.

Table 1 below gives exemplary precoder sequences derived from thegenerator matrix G for transmission ranks from 1 to 4, where G_([n) ₁_(. . . n) _(K) _(]) denotes a filter matrix with the columns n_(i) . .. n_(K) taken from G.

TABLE 1 Precoder Sequences for Transmission Ranks 1 to 4 Rank PrecoderSequence 1 G_([1]) G_([2]) G_([3]) G_([4]) 2 G_([12]) G_([34]) G_([13])G_([24]) G_([14]) G_([23]) 3 G_([123]) G_([124]) G_([134]) G_([234]) 4 GAs seen in Table 1, the period of the precoder sequence for eachpossible transmission rank equals the number of all possible columncombinations, and each precoder sequence uses each possible precodingfilter exactly once. For transmission rank 2, the precoding filters arepaired and ordered such that the full vector space is covered by eachpair. That is the first two filters form a first pair, the next twofilters form a second pair, and so forth. The pairing is advantageouswhere the channel varies significantly within one resource block becauseit is beneficial for uniformity to cover the full vector space with aslittle channel variations as possible.

FIG. 4 illustrates an exemplary method 150 of transmitting signals froma multiple antenna transmitter 200. The transmit controller 110determines the rank of the channel and selects the desired transmissionrank (block 152). The channel rank may be determined in a conventionalmanner. The transmission rank is chosen to use as many transmissionlayers as the channel can support. Once the transmission rank isdetermined, the transmit controller 110 indicates the selectedtransmission rank to the transmit signal processor 120. The transmitsignal processor 120 selects the precoder sequence corresponding to thetransmission rank (block 154). As previously noted, the precodersequence for each possible transmission rank is known a priori to thetransmitter 100. The precoder sequence determines the set of precodingfilters to be used and the order in which the precoding filters areapplied. The precoding filters may be pre-stored in memory.Alternatively, the generator matrix may be stored in memory and theprecoding filters may be constructed on-the-fly from the generatormatrix after the transmission rank is determined. With the selected setof precoding filters, the transmit signal processor 120 precodes theinformation symbols (block 156) and transmits the precoded symbols(block 158). During the precoding, the transmitter 100 changes or cyclesthe precoding filters while traversing the OFDM resource grid. Forexample, the precoding filter may be kept fixed for a set of one or moresubcarriers and then changed for the next set of subcarrier according tothe selected precoder sequence.

FIG. 5 illustrates an exemplary receive signal processor 220 accordingto one embodiment of the invention for decoding signals transmitted bythe transmitter 100. The receive signal processor 220 comprises areverse layer mapping unit 222, a precoder 224, and a plurality of FastFourier Transform (FFT) processors 226. The FFT processors 226 mayperform a Discrete Fourier Transform or the inverse operation. Thesignal received at each antenna port 228 is processed by a correspondingFFT processor 226. The output from each FFT processor 226 is input tocombiner 224. The combiner 224 combines the outputs from each FFTprocessor 226 and outputs a received symbol stream corresponding to eachtransmitted layer. The combiner 224 uses a set of combining filters thatare selected based on the transmission rank and which match theprecoding filters used by the transmitter 100. The combiner 224 cyclesthrough the set of combining filters, using a different one of thecombining filters during different combining intervals. The combiningintervals at the receiver correspond to precoding intervals at thetransmitter 100. The symbol streams output from the combiner 224 arethen combined into a single received symbol stream by the reverse layermapping unit 222. This symbol stream may be subject to furtherprocessing, such as rate-dematching, soft buffer combining, and turbodecoding.

FIG. 6 illustrates an exemplary method 250 of receiving signals from amultiple antenna transmitter 200. The receive controller 210 determinesthe transmission rank used by the transmitter 100 (block 252). Thetransmitter 100 may inform the receiver 100 of the transmission rank ina signaling message. Alternatively, the receiver may determine thetransmission rank itself based on the channel rank. Once thetransmission rank is determined, the receive controller 210 indicatesthe selected transmission rank to the receive signal processor 220. Thereceive signal processor 220 selects a combining sequence correspondingto the transmission rank, which is known a priori to the receiver 200(block 254). The combining sequence determines the set of combiningfilters to be used and the order in which the combining filters areapplied. The combining filters may be pre-stored in memory.Alternatively, the generator matrix may be stored in memory and thecombining filters may be constructed on-the-fly from the generatormatrix after the transmission rank is determined. With the selected setof combining filters, the receive signal processor 220 combines theoutput of each FFT processor 226 to generate a symbol streamcorresponding to each layer (block 256). The symbol streamscorresponding to each layer are then output to the reverse layer mappingunit 222 (block 258).

The invention offers an efficient way to support open-loop MIMOtransmission particularly targeting rank two or higher ranktransmissions. Computational complexity for demodulation and CQIcomputation in the UE is reduced and the feasibility of interferencerejection is improved compared to existing solutions. The increaseduniformity of the transmission in the spatial domain improves therobustness of the open-loop MIMO mode. The use of a single generatormatrix may result in considerable complexity savings as many of thecomputations for CQI and demodulation may be reused across severaldifferent ranks and when identifying the characteristics of theinter-cell interference.

The present invention may, of course, be carried out in other ways thanthose specifically set forth herein without departing from essentialcharacteristics of the invention. The present embodiments are to beconsidered in all respects as illustrative and not restrictive, and allchanges coming within the meaning and equivalency range of the appendedclaims are intended to be embraced therein.

What is claimed is:
 1. A method of spatially precoding data in a channel independent way, said method comprising: selecting a transmission rank based on a transmission rank of a communication channel; selecting a set of one or more precoding filters equivalent to column subsets of a single generator matrix based on said selected transmission rank; and precoding data for transmission to a remote device using different ones of said precoding filters during different precoding intervals in a precoding period of a predetermined precoding sequence; wherein selecting a set of one or more precoding filters comprises selecting one or more columns of the generator matrix for each precoding filter, and wherein the number of columns selected for each precoding filter equals the transmission rank; and wherein the ordering of said precoding filters in said predetermined precoding period is such that, for a selected transmission rank, non-overlapping fixed-size groups of consecutively used precoding filters do not have any columns in common.
 2. The method of claim 1 wherein selecting a set of one or more precoding filters further comprises selecting a set of precoding filters such that the minimum subspace distance between any two precoding filters in the set of precoding filters is maximized according to a predetermined distance criterion.
 3. The method of claim 2 wherein the distance criterion comprises at least one of chordal, projection two-norm, or Fubini-Study distance.
 4. The method of claim 1, wherein selecting a set of one or more precoding filters comprises selecting the set of precoding filters such that each column of the generator matrix is used the same number of times.
 5. The method of claim 1, wherein said generator matrix comprises a QPSK alphabet generator matrix.
 6. The method of claim 5 wherein the number of precoding intervals in said predetermined precoding period equals the number of possible combinations of columns for the selected transmission rank, and wherein each precoding filter in said set comprises one of said possible combination of columns from said generator matrix.
 7. The method of claim 1 applied to an Orthogonal Frequency Division Multiplexing (OFDM) system wherein precoding data for transmission to a remote device comprises traversing an OFDM resource block in an alternating pattern during said predetermined precoding period.
 8. The method according to claim 7, wherein the alternating pattern comprises a frequency first order from top to bottom in even-numbered symbol periods and a frequency first order from bottom to top in odd-numbered symbol periods, or vice versa.
 9. The method claim 1, wherein precoding data for transmission to a remote device comprises precoding data using each precoding filter in said set of precoding filters the same number of times over said predetermined precoding period.
 10. A transmitter for transmitting spatially precoding data in a channel independent way comprising: a transmit controller configured to determine a transmission rank of a communication channel and to select a set of one or more precoding filters equivalent to column subsets of a single generator matrix based on said transmission rank; and a transmit signal processor including a precoder configured to precode said 30 data for transmission using different ones of said precoding filters during different precoding intervals in a precoding period of a predetermined precoding sequence; wherein the transmit signal processor is further configured to select one or more columns of the generator matrix for each precoding filter, and wherein the number of columns in each precoding filter equals the transmission rank; and wherein the transmit signal processor is further configured to apply the precoding filters in said predetermined precoding period such that, for a selected transmission rank, non-overlapping fixed-size groups of consecutively used precoding filters do not have any columns in common.
 11. The transmitter of claim 10 wherein the transmit signal processor is further configured to select the set of filters that maximizes a minimum subspace distance between any two precoding filters in the set according to a predetermined distance criterion.
 12. The transmitter of claim 11 wherein the distance criterion comprises at least one of chordal, projection two-norm, or Fubini-Study distance.
 13. The transmitter of claim 11, wherein the transmit signal processor is further configured to select the set of precoding filters such that the selected set of precoding filters uses each column of the generator matrix the same number of times.
 14. The transmitter of claim 13 wherein the number of precoding intervals in said predetermined precoding period equals the number of possible combinations of columns for the selected transmission rank, and wherein each precoding filter in said set comprises one of said possible combination of columns from said generator matrix.
 15. The transmitter of claim 10 for an Orthogonal Frequency Division Multiplexing (OFDM) system wherein the transmit signal processor is further configured to precode data for transmission to a remote device by traversing an OFDM resource block in an alternating pattern during said predetermined precoding period.
 16. The transmitter according to claim 15, wherein the alternating pattern comprises a frequency first order from top to bottom in even-numbered symbol periods and a frequency first order from bottom to top in odd-numbered symbol periods, or vice versa.
 17. The transmitter of claim 10, wherein said generator matrix comprises a QPSK alphabet generator matrix.
 18. The transmitter of claim 10, wherein the transmit signal processor is further configured to precode data using each precoding filter in said set of precoding filters the same number of times over said predetermined precoding period.
 19. A method of receiving spatially precoded data, precoded in a channel independent way, received from a remote device, said method comprising: determining a transmission rank applied at a transmitter; said method characterized by: selecting a set of one or more combining filters based on said transmission rank, wherein said combining filters corresponding to a set of precoding filters are equivalent to column subsets of a single generator matrix; and combining said spatially precoded data using different ones of said combining filters during different combining intervals in a combining period of a predetermined combining sequence; wherein selecting a set of one or more combining filters comprises selecting one or more columns of the generator matrix for each combining filter, and wherein the number of columns selected for each combining filter equals the transmission rank; and wherein the ordering of said combining filters in said predetermined combining period is such that, for a selected transmission rank, non-overlapping fixed-size groups of consecutively used combining filters do not have any columns in common.
 20. The method according to claim 19, wherein selecting a set of one or more combining filters comprises selecting the set of combining filters such that each column of the generator matrix is used the same number of times.
 21. The method of claim 19 applied to an Orthogonal Frequency Division Multiplexing (OFDM) system wherein combining said spatially precoded data comprises traversing an OFDM resource block in an alternating pattern during said predetermined combining period.
 22. The method according to claim 21, wherein the alternating pattern comprises a frequency first order from top to bottom in even-numbered symbol periods and a frequency first order from bottom to top in odd-numbered symbol periods, or vice versa.
 23. The method of claim 19, wherein the number of combining intervals in said predetermined combining period equals the number of possible combinations of columns for the selected transmission rank, and wherein each combining filter in said set comprises one of said possible combination of columns from said generator matrix.
 24. The method of claim 19, wherein each combining filter in said set of combining filters is used the same number of times over said predetermined combining period.
 25. A receiver for receiving spatially precoded data, precoded in a channel independent way, said receiver comprising: a receive controller configured to determine a transmission rank applied at a transmitter and to select a set of one or more combining filters wherein said combining filters corresponding to a set of precoding filters are equivalent to column subsets of a single generator matrix based on said transmission rank; and a receive signal processor including a combiner configured to combine said spatially precoded data using different ones of said combining filters during different combining intervals in a combining period of the predetermined combining sequence; wherein said receive signal processor is further configured to select one or more columns of the generator matrix for each combining filter, and wherein the number of columns selected for each combining filter equals the transmission rank; and wherein the ordering of said combining filters in said predetermined combining period is such that, for a selected transmission rank, non-overlapping fixed-size groups of consecutively used combining filters do not have any columns in common.
 26. The receiver of claim 25, wherein said receive signal processor is further configured to select the set of combining filters such that each column of the generator matrix is used the same number of times.
 27. The receiver of claim 25, applied to an Orthogonal Frequency Division Multiplexing (OFDM) system, wherein the receive signal processor is configured to combine said precoded data while traversing an OFDM resource block in an alternating pattern during said predetermined combining period.
 28. The receiver according to claim 27, wherein the alternating pattern comprises a frequency first order from top to bottom in even-numbered symbol periods and a frequency first order from bottom to top in odd-numbered symbol periods, or vice versa.
 29. The receiver of claim 25, wherein the number of combining intervals in said predetermined combining period equals the number of possible combinations of columns for the selected transmission rank, and wherein each combining filter in said set comprises one of said possible combination of columns from said generator matrix.
 30. The receiver of claim 25, wherein each combining filter in said set of combining filters is used the same number of times over said predetermined combining period. 